Sensors are in conventional use for measuring the angular rate of rotation of wheels or gears. Such sensors generate an analog signal, substantially sinusoidal, with a frequency proportional to the angular rate. Representative of these sensors is the well-established variable reluctance sensor that measures the rate of change in flux as appendages such as teeth on the wheel or gear pass the sensor. Ideally, the rate of change in flux over time is sinusoidal and is centered around a fixed reference voltage.
Applications of such sensors are widespread in the automotive field, where sensors are used to measure the rate of rotation of the crankshaft or camshaft of the vehicle engine, or may be used to measure the rate of rotation of the vehicle wheels. As such vehicles typically employ digital controllers, any wheel speed information of interest to the controller must by converted to digital form.
The typical digital controller will require a binary substantially square wave signal within some predetermined voltage range, such as zero to five volts. Of course, the frequency of the digital signal should be proportional to the frequency of the analog signal from which it was generated. Furthermore, it is desirable to maximize resolution of the signal, which is provided by approaching a 1:1 ratio between the analog and digital signal frequencies.
Converters are available commercially that compare the analog signal to fixed voltage thresholds and switch the digital equivalent between its binary levels when the analog signal crosses the thresholds. Such systems may reliably generate a digital equivalent of the analog signal in the ideal case, such as when the analog signal is centered around a fixed reference voltage, but when the signal drifts, for instance when the a.c. components of the signal are not centered around a fixed voltage level, such as zero volts, the system may miss one or many oscillations of the signal. The reference of the analog signal may vary so much as to take the entire signal outside the fixed threshold voltage range, so that the circuit does not see a crossing of the thresholds and consequently does not switch the digital equivalent.
As described, the output of an ideal variable reluctance sensor is sinusoidal and is centered around a fixed reference voltage. Practically however, sensors may deviate substantially from the sinusoidal model, and may be centered around a reference that varies significantly and unpredictably. So great may be the variation in the reference voltage that the sensor output oscillations may not cross a fixed reference relied on in a conventional digital converters. As such, analog oscillation information will be lost and the digital signal generated by the converter will be less accurate. Variations in the reference voltage may result from a wheel or gear that does not rotate about its center, or from teeth that fluctuate in size. Such defects may not be easy to diagnose, and may not be easily or inexpensively repaired.
Accordingly, what is needed is a method and apparatus for converting the analog output of a wheel or gear speed sensor to a digital value that is usable by a digital controller and that is insensitive to the unpredictable variations in the reference voltage of the analog signal.